Cathode ray tubes



Sept. 6, 1955 R. c. HALL 2,717,324

CATHODE RAY TUBES Filed May l, 1951 love/:fof

RONALD CHARLES HALL United States Patent O :pas

CATHDDE RAY TUBES Ronald Charles Hall, Cowley, England, assigner to Electric & Musical Industries Limited, Hayes, England, a company of Great Britain Application May 1, 1951, Serial No. 223,857

Claims priority, application Great Britain May 4, 1950 6 Claims. (Cl. 313-84) This invention relates to cathode ray tubes employed for example in television receivers. lt is very desirable that the beam of the tube should be accurately centred with respect to the fluorescent screen of the tube and various proposals have been made for providing means whereby adjustment of the normal, that is to say the undeflected, position of the beam may be effected. For example it has been proposed in the case of a tube provided wlth a permanent magnet focussing lens to centre the beam, if necessary, by tilting the permanent magnet lens so that it is no longer at right angles to the neck of the tube, or by laterally displacing the permanent magnet lens so that it is no longer coaxial with the tube neck. Such prior arrangements, however, all suffer from the disadvantage that when the beam has been centred the permanent magnet lens is not coaxially disposed in relation to the undefiected beam so that the spot is distorted from the desired circular shape, the distortion being most pronounced when the spot is at the edges of the scanned raster. In addition, the spot tends to be defocussed when the focussing lens is moved to centre the beam. The object of the present invention is to provide beam centring means which are not subject to these disadvantages.

Accordingly the invention provides, in combination with a cathode ray tube of the type having a neck portion along which a beam of electrons can be projected axially thereof, a magnetic focussing system for focussing said beam, means mounting said focussing system around said neck portion, substantially magnetically neutral ferromagnetic means disposed around said neck portion in a position physically spaced from said focussing system but magnetically coupled thereto, means mounting said ferromagnetic means independently of said focussing system mounting means, said means mounting said ferromagnetic means including means for effecting tilting movement of said ferromagnetic means so that said ferromagnetic means can be adjusted to positions other than normal to the axis of said neck portion to enable the direction of said beam to be varied subsequent to its passage through the magnetic field of said focussing system.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be more fully described with reference to the accompanying drawings, in which:

Figure l illustrated diagrammatically a cathode ray tube according to the invention, and

Figure 2 is an end view of ferro-magnetic means shown in Figure l.

In Figure l, a cathode ray tube 1 has assembled on the neck thereof an assembly, indicated generally at 2, which comprises a magnetic focussing system 3, a beam deflecting means 4, and ferro-magnetic means 5 for centring the normal, that is to say, the undeflected, position of the beam. The focussing system 3 may be of the permanent magnet type, or of the electro-magnetic type, or it may comprise a combination thereof, the essential requirement for the purpose of the invention being that the system 3 2,717,324 Patented Sept. 6, 1955 ice is one in which stray magnetic flux therefrom is present to co-operate with the ferro-magnetic means 5 as referred to hereinafter.

The ferro-magnetic means 5 is shown in Figure 2 in the form of a circular annulus surrounding the neck of the tube 1, and having a central aperture of a size sufficient to provide a suitable clearance between it and the tube neck in order that the annulus may be capable of tilting movement so that it can be adjusted to positions which are not normal to the axis of the neck portion of the tube 1. The ferro-magnetic means 5 is composed of any suitable material such that it provides a path of low magnetic reluctance for stray magnetic flux from the focussing system 3, and satisfactory results have been achieved with 1 an annulus of mild steel.

If the ferromagnetic means 5 is disposed in a plane approximately normal to the axis of the tube 1 and assuming that the stray flux from the focussing system 3 is symmetrical in relation to said axis, then the magnetic field in the neck of the tube 1 in the region of the ferromagnetic means 5 will have no resultant transverse components and consequently the path of the beam will be substantially unaffected. If, however, the ferromagnetic means 5 is tilted so that it lies in a plane other than normal to said axis, then said magnetic field will have a resultant transverse component the magnitude of which depends on the angle of tilt, and the path of the beam will be varied accordingly. in a cathode ray tube of the type normally used in television receivers, provided with a beam centering arrangement according to the invention, relatively large variations in the direction of the beam can be accomplished even when the focussing field, and hence the stray flux emanating therefrom is relatively weak.

The assembly 2 comprising the focussing system 3, the deflecting means 4 and the ferro-magnetic means 5 is illustrated in Figure l as a unitary assembly in which the components are held in position by a suitable framework made of brass or other non-magnetic material, such that the ferromagnetic means S is disposed between the focussing system 3 and the defiecting means 4 which are in substantially coaxial relationship. Longitudinal strips 6 and 7 arranged in spaced parallel relation have their forward ends secured to L-shaped members 8 and 9 by means of bolts 1li and 11, the members 8 and 9 being in turn secured to the diametrically projecting ears of a band 12 closely encircling the deflecting means 4. The rear ends of strips 6 and 7 are secured in a similar manner by means of bolts 13 and 14 to further L-shaped members 15 and 16 which serve to hold the focussing system 3. The bolts 13 and 14 are provided with knurled heads to facilitate manual adjustment and the apertures in the strips 6 and 7 through which bolts 13 and 14 pass are elongated, whereby the focussing system 3 can be shifted along the neck of tube 1 to a position at which the beam is correctly focussed and at which the focussing system 3 is disposed with reference to the ferromagnetic means 5, such that the latter is subject to stray magnetic ux, but is sufficiently far from the focussing system 3 substantially to prevent any interference with the proper functioning thereof. If required both the deflecting means 4 and the ferromagnetic means 5 can be mounted for positional adjustment longitudinally of the tube neck in a manner similar to that described for the focussing system 3.

As shown in Figure 2, the annulus comprising the ferromagnetic means 5 is mounted in a frame 17 which is also composed of a suitable non-magnetic material such as brass. Extending through apertures in opposite sides of the frame 17 are two bolts 18 and 19 the inner ends of which are bifurcated to embrace and hold diametrically opposed peripheral portions of the annulus. By this means the annulus is capable of being tilted about a diametric axis in relation to the frame 17, and the outer ends `of bolts 18 and 19 yare provided with nuts 20 and 21 whereby the annulus can be firmly secured in a desired position.

The frame 17 is adjustably secured to the strips 6 and 7 and is disposed between the focussing system 3 and the deiiecting means 4i. Bolts 22 and 23 together with nuts ,24 and 25 are arranged with spacing washers 26 and 27, so that the frame 17 is spaced from the strips 6 and 7, and can be tilted together with the ferromagnetic means 5 about an axis which is perpendicular to that about which the annulus is tiltable with reference to the frame 17. Thus lthe annulus is mounted for independent tilting movements about two mutually perpendicular axes both of which are normal to the axis of the neck of the tube 1.

Itis convenient to arrange that tilting of the ferromagnetic means 5 about one axis serves to effect beam centering in a horizontal plane, whereas tiiting about the other axis results in vertical shifting of the beam.

It has been found that the ferro-magnetic means 5 has no appreciable effect on the correct functioning of the focussing system 3, and since the latter can be maintained vcoaxial with the tube axis, spot distortion and defocus sing which tends to result when the focussing system is not coaxial with the tube axis is eliminated.

As shown in Figure 1 the disposition of the components of the assembly' 2 is such that the ferro-magnetic means S serves to reduce the penetration of magnetic flux from the defiecting means 4 into the focussing system 3 and thereby assists in reducing or eliminating defocussing of the spot which tends to occur as the spot is deflected.

The invention is particularly useful when used in conjunction with a cathode ray tube of the type usually employed in a television receiver, and if in such a tube it is found that the rectangular raster produced by the detiecting means is not positioned symmetrically on the uorescent screen of the tube, the invention provides a simple and easily eected remedy. The nuts 2S, 21, 24, and 25 are loosened and the ferro-magnetic means 5 can then be tilted to bring the raster into the centre of thc fluorescent screen, whercafter the nuts 20, 21, 24, and 25 are tightened to hold the ferro-magnetic means 5 in the correct position of adjustment.

The invention is not limited to any particular shape or form for the ferromagnetic means 5, since instead of a circular annulus as described, a square plate with an' aperture of suitable size could be employed. In addition it is not necessary for the ferro-magnetic means to form a closed magnetic circuit as in the case of an annulus, and in an alternative arrangement discrete ferro-magnetic members can be arranged symmetrically around the neck of the tube 1, for example, by being attached to a suitable non-magnetic support which is capable of tilting movement inta manner similar to the afore-mentioned annulus.

ln the permanent magnet type focussing system 3 illustrated in Figure 1, the pole pieces 28 and 29 are of equal size, but if one of the pole pieces is of smaller diameter than the other, then it is preferable to arrange for the ferro-magnetic means 5 to face the smaller pole piece thereby to utilise a larger stray magnetic field than would be present if the ferro-magnetic means 5 were facing the larger pole piece.

, Qther methods of mounting the ferro-magnetic means 5 to give the required adjustments can be used. For example, provision may be made for tilting movement about only one axis, and in this case the ferro-magnetic means 5 is arranged to be rotatable about the neck of the tube, so that by a suitable combination of tilting and rotating the beam can be accurately centred.

l claim:

1. ln combination with a cathode ray tube of the type having a neck portion along which a beam of electrons can be projected axially thereof, a magnetic focussing system for focussing said beam, means mounting said focussing system around said neck portion, substantially magnetically neutral ferromagnetic means disposed around said neck portion in a position physically spaced from said focussing system but magnetically coupled thereto, means mounting said ferromagnetic means inependently of said focussing system mounting means, said means mounting said ferromagnetic means including means for effecting tilting movement of said ferromagnetic means so that said ferromagnetic means can be adjusted to positions other than normal to the axis of said neck portion to enable the direction of said beam to be varied subsequent to its passage through the magnetic field of said focussing system.

2. 1n the combination according to claim 1, means for effecting independent movements of said ferromagnetic means about two axes mutually perpendicular and normal to the axis of said neck portion.

3. in the combination according to claim 1, a circular annulus of ferromagnetic material constituting said ferromagnetic means.

4. In combination with a cathode ray tube of the type having a neck portion along which an electron beam can be projected axially thereof, a magnetic focussing system for focussing said beam, deflecting means for deecting said beam, substantially magnetically neutral ferromagnetic means, a framework embracing said neck portion, means coupled to said framework mounting said focussing system around said neck portion, means coupled to said framework mounting said deflecting means around said neck portion, means coupled to said framework independent of said focussing system mounting means mounting said ferromagnetic means around said neck portion between said deecting means and said focussing system in a position physically spaced from said focussing system but magnetically coupled thereto, said means mounting said ferromagnetic means inf cluding means for effecting tilting movement of said ferromagnetic means so that said ferromagnetic means can be adjusted to positions other than normal to the axis of said neck portion to enable the direction of said beam to be varied subsequent to its passage through the magnetic field of said focussing system.

5. In the combination according to claim 4, means for effecting independent movements of said ferromagnetic means about two axes mutually perpendicular and normal to the axis of said neck portion.

6. In the combination according to claim 4, a circular annulus of ferromagnetic material constituting said ferromagnetic means.

References Cited in the tile of this patent UNiTED STATES PATENTS 2,102,421 Kuehni Dec. 14, 1937 2,185,857 Lubszynski et al. Jan. 2, 1940 2,234,720 De Tar Mar. 11, 1941 2,501,516 Holden Mar. 21, 1950 2,513,929 Gethmann July 4, 1950 2,533,689 Quam Dec. 12, 1950 2,539,156 Ostreicher Jan. 23, 1951 

